This invention relates to magnetically shielded rooms, and, more particularly, to the construction of such a room having a sliding door for access to the room.
In biomagnetometry, magnetic fields produced by the human body are measured and studied both to diagnose ailments and to understand the functioning of the body. The magnetic fields of greatest interest are produced by the brain, and the strength of these fields produced by the brain are on the order of 1/10,000,000 of the strength of the magnetic field of the earth. There are several approaches to isolating the magnetic field of the brain from the ambient magnetic field of the earth and other sources. One approach is to provide sophisticated electronic filtering that separates the varying fields of the brain from the steady field of the earth. Such filtering electronics is expensive and not fully effective.
Another approach is to place the person whose magnetic field is under study into an isolating enclosure which shields the person and the magnetic field detector from external electromagnetic and magnetic fields, so that the biomagnetic field produced by the person constitutes a greater fraction of the total field within the enclosure. Such an enclosure is known in the art as a "magnetically shielded room" or MSR. The use of a magnetically shielded room in conjunction with some lesser level of electronic filtering than used in the absence of the MSR produces the best results for measurements of biomagnetic signals obtained to date.
One of the earliest magnetically shielded rooms is disclosed in U.S. Pat. No. 3,557,777. Although the configurations of the patented design and those of current commercial products vary somewhat and employ various modifications to improve performance and ease of construction, the basic design of a magnetically shielded room remains unchanged. Such a room includes four walls, a floor, a ceiling, and a door in one of the walls. This enclosure is structured to have a "continuous" electrically conductive layer that excludes radio frequency fields from the interior, and a "continuous" layer of a high magnetic permeability material that excludes magnetic fields from the interior of the MSR.
In this description, the word "continuous" is highlighted, because the layers cannot be truly continuous due to the need to construct the enclosure from sheets of finite size, because there are electrical feedthroughs to permit monitoring of the instrumentation in the room, and because there is a door in the enclosure to permit access to the interior of the MSR. Even tiny separations between sections of the layers can permit external fields to leak into the interior of the MSR. Many of the design improvements made to MSRs are aimed at removing field leakage paths while retaining reasonable weight, cost, and ease of construction of the MSR. In one approach, an additional layer of the high magnetic permeability material is added within the structure with its seams offset from those of the outer layer, in the hope of minimizing magnetic field leakage.
To date, magnetically shielded rooms have been made with swinging doors that seal to the facing wall in the manner of a refrigerator or cold-storage locker. This type of door provides moderately good sealing of the electrically conductive and high magnetic permeability layers, when there is one layer of each type, although it is difficult to attain good sealing at the bottom of the door along the threshhold of the doorway. When there is one electrically conductive layer and two layers of the high magnetic permeability material, it is more difficult to attain simultaneous sealing of all of the layers. There has been presented no good design for a swinging door when a second layer of the electrically conductive material is added, for a total of four layers.
Swinging doors are heavy, and may weigh from 800 to 1000 pounds, depending upon the details of construction. Swinging doors are cumbersome to use, requiring a larger "footprint" for the MSR than is required by the plan view of the room itself. Since the MSR is usually constructed within a room of a hospital or laboratory building, with the electronic and analytical support equipment in that exterior room, the operations can be quite cramped because of the space that must be allotted to the opening of the door. Swinging doors can pose some safety hazards in operation, particularly in a cramped environment.
There is therefore a need for a better approach to entry into a magnetically shielded room. Such an approach must permit excellent sealing of the shielding layers to exclude external fields, and desirably would be more convenient to use than the existing approach. The present invention fulfills this need, and further provides related advantages.